The present invention relates to a vane-type variable displacement pump suitable for use with a pressurized fluid utilizing equipment such as a power steering unit reducing operating force of an automotive handle.
A constant capacity type vane pump, which is directly driven by an automotive engine, is generally used as a pump for the power steering unit. However, the constant capacity type pump has an inherent feature that the quantity of flow discharged from the pump is increased in proportion to the increase of the revolution number of the engine, contrary to the demand wherein the power steering unit produces a large assisting force when the vehicle stops or runs at a lower speed, and a small assisting force when the vehicles runs at a high speed, in view of the running stability and steering feeling.
Thus, the conventional constant capacity type pump is arranged to have a sufficiently large pump chamber to produce a predetermined power assisting force in a low revolution number area and is further provided with a flow control valve through which a part of or most of the discharged flow is released to a tank side in a high revolution area. With such arrangement and by the virtue of the flow control valve, the constant capacity type pump can maintain the quantity of flow supplied to the power steering unit at a constant valve regardless of the revolution number of the pump or can control the quantity of flow supplied to the power steering unit at the low revolution area smaller than that in the high revolution area.
This constant capacity type, however, suffers from a problem in that the flow control valve is indispensable for controlling the pump discharge flow quantity less than a constant quantity, which results in the increase of the components parts for constituting the necessary pump construction. Further, the conduit or passage arrangement is made complicated, to unavoidably increase the size of the pump and the production cost therefor.
Furthermore, the quantity of wasteful flow released from the pump discharge side to the tank side through the flow control valve without being transmitted to the power steering unit is increased as the revolution number of the pump is increased, to increase the wasteful drive force and to deteriorate the energy converting efficiency. That is to say, loss of energy is increased as the engine rotates at the higher revolution number and produces the higher pressure, for example, when the vehicle runs at high speed or comes up a slope.
Moreover, releasing the pump discharge side fluid to the tank side through the above-mentioned flow control valve causes the temperature increase, which results, in some case, in the change of the steering characteristic, the lowering of the pump volume efficiency due to the internal leakage, the seizing of the rotor and the cam ring, the adverse effect onto the sealing members or the like. Therefore, a cooling means such as cooling pipe must be provided for some kind of vehicles, which causes further increase in the production cost.
For these reasons, as a hydraulic or oil pressure pump used for a power steering unit, prior art proposes, in place of the constant capacity type pump, the use of a variable capacity or displacement type pump wherein the pump discharge side flow quantity is changed to decrease in a step like manner as the revolution number increases. Such variable capacity type pump is disclosed, for instance, in Japanese Patent Kokai Publications Sho. 53-130505 and Sho. 56-143383. The proposed variable capacity type pump dispenses with the flow control valve and prevents the increase of the lost drive force to improve the energy efficiency. Moreover, since there is no released flow to the tank side, it is possible to solve the conventional problem in that the fluid temperature is increased due to the released flow. Thus, the variable capacity type pump is also advantageous over the constant capacity type pump in preventing the problem of the internal leakage of the pump, lowering the volume efficiency and so on.
Hereinafter, the variable capacity or displacement type pumps proposed by the publications are described.
The variable displacement type pump disclosed in Japanese Publication No. Sho. 53-130505 is constructed such that an eccentric amount between a center of a rotor of a vane pump and a center of a hollow cam surface to which the vanes are slid is made variable, and further a communication area of a variable orifice provided in a pump discharge side conduit is designed to be decreased as the eccentric amount of the cam ring having the hollow cam surface is decreased. With such construction, the movement of the cam ring is controlled utilizing a pressure difference between the front and the rear of the variable orifice, to thereby decrease the discharge flow quantity in association with the increase of the rotor revolution number.
The variable displacement type pump disclosed in Japanese Publication No. Sho. 56-143383 is constructed such that a cam ring is made movable within a pump casing, and a pair of control chambers are formed in a space between the cam ring and the casing. Pressures in front and rear portions of an orifice provided in a discharge passage are transmitted to the respective control chambers, to effectuate the pressure difference directly on the cam ring to move the cam ring against a biasing force of a spring, whereby the volume of a pump chamber is varied to perform the discharge flow quantity control.
However, since the cam ring is simply held within the pump housing so as to be movable linearly, and is driven to be moved by the pressure difference between the front and rear portions of the orifice provided directly or indirectly on the discharge passage, these variable displacement type pumps still raise problems not only in machining and assembling properties but also in operational reliability and durability. Thus, the practical applicability is poor in use.
Japanese Patent Kokai Publication Sho. 58-93978 and Japanese Utility Model Kokoku Publication Sho 63-14078 also disclose a variable displacement type pump wherein a cam ring is disposed within a pump housing to be linearly movable in a radial direction thereof, a rotor is rotatably accommodated within the cam ring to form a pump chamber therebetween, and the cam ring is driven to be moved relative to the rotor by the pressure difference between front and rear portions of an orifice provided in a pump discharge passage. In the pump, the flow passage area of the orifice is made variable in accordance with the amount of the eccentric displacement of the cam ring relative to the rotor to obtain a desired quantity of discharge flow.
This type of pump, in particular that disclosed in Publication 078 is constructed such that a control pin having a small diameter portion is interposed between an inner wall of the pump housing and an outer periphery of the cam ring movable within the pump housing, and a variable orifice is formed by the combination of the small diameter portion of the control pin and the control surface of the cam ring outer periphery. By forming the orifice such that the pressures in front of and behind this orifice are applied onto the cam ring to displace the cam ring, and the opening area of this orifice is made decreased in association with the decrease of the eccentric displacement of the cam ring, a desired quantity of discharge flow is obtained.
However, in the conventional arrangements, the orifice construction provided in a portion of the pump discharge passage for moving the cam ring is made complicated, and it is difficult to provide sufficient machining accuracy in each portion. Thus, these conventional arrangements also raise problems in machining and assembling, and are still insufficient in operational reliability of variable orifice portion.
That is to say, in the conventional arrangements mentioned above, the variable orifice for moving the cam ring is merely provided on a portion of the pump discharge side passage. It is impossible for this construction to produce a large pressure difference between the front and rear portions of the variable orifice to obtain sufficient pressure difference for enabling the movement of the cam ring within the pump housing. Consequently, the cam ring can not be expected to be moved to present a desired state in accordance with the pump revolution number. Thus, the variable capacity type pump as disclosed has a possibility that it does not surely perform the desired function of a variable capacity type pump.
In particular, in the above-noted conventional arrangements, if dust or the like enters an operation oil which is a control fluid, or a pump housing is deformed under a high pressure, operation performance where the cam ring is moved by the front and rear pressures of the orifice is like to become unstable. More specifically, the cam ring forms at its side face only a slight clearance in order to prevent internal leakage as well known, so that the increase of the sliding resistance due to the clogging of the dust hinders smooth displacing motion. Thus, there is a possibility that a desired flow control can not be obtained.
If the pressure difference between the front and rear of the orifice is set greater in order to conquer such resistance, it is impossible to obtain a low-consumption feature applicable to the power steering unit with sufficient performance, which is a primary objective of the variable displacement type pump. Therefore, some means attempting to solve this problem has been required in the art.
Further, according to the above-noted arrangement, in case where the movable cam ring is fixingly retained within the pump body so as not to slide any more or is fixingly caught at any position, the increase of the pump revolution number results in the increase of the quantity of the pump discharge flow in proportion thereto (See FIG. 11B).
In such case, the large quantity is discharged to the power cylinder side to suddenly lighten the operation feeling of the steering handle. In particular, under a high speed or high revolution number condition, this phenomenon is severe, and is likely to raise a problem in safety. Some means for solving this problem has also been required in the art.